Studies will be made of the effects of drugs on transmitter release at the neuromuscular junction. Of primary concern is the analyses of the changes produced by drugs in p (probability of release), n (available stores) and transmitter mobilization. Whereas tetraethylammonium causes an increase in p and has no effect on n, an analog of hemicholinium casuses a decrease in n but has no effect on p. Unlike the results obtained by others, lithium has been shown to increase transmitter release during nerve stimulation and to cause a calcium-dependent increase in the frequency of miniature endplate potentials. Only partial substitution of lithium for sodium is required to show these actions of lithium. Lithium alters the frequency-facilitation curve by increasing quantal content without an effect on mobilization. Lithium increases p and has no effect on n. It is unlikely that these effects of lithium are related solely to depolarization of the nerve terminals.